Method and apparatus for separating oxygen from a gaseous mixture

ABSTRACT

A process and apparatus is provided for separating oxygen from a mixture of gases such as air. The apparatus includes an electrochemical cell that includes a cathode, an anode and an electrolyte. Oxygen in the air is reduced to the superoxide ion (O 2   - ) at the cathode; the superoxide ion is transported across the cell through the electrolyte; and the superoxide ion is then reoxidized to oxygen at the anode and collected.

FIELD OF THE INVENTION

This invention relates to a process and apparatus useful for separatingoxygen from a mixture of gases by reducing the oxygen to its superoxideion (O₂ ⁻), transporting the superoxide ion from the reducingenvironment to an oxidizing environment and reoxidizing the superoxideion to oxygen. More particularly, this invention relates to a processfor separating oxygen from air in an electrochemical cell, whereinoxygen is reduced to the superoxide ion at the cathode; the superoxideion is transported across the cell through an electrolyte; and thesuperoxide ion is reoxidized to oxygen at the anode.

BACKGROUND OF THE INVENTION

Oxygen gas has many uses. For example, it can be used for treatment ofpatients in the medical field, for various industrial processes, and forbreathing in an environment in which oxygen is deficient. As a result ofthe variety of uses for oxygen gas, there is currently a substantialdemand for such gas and also for a process by which it can be producedeconomically, efficiently and safely. Preferably such a process can becarried out in relatively large units and also in relatively smallunits, e.g. portable units.

One process that is presently used to produce oxygen gas is electrolysisof water. There are, however, several problems associated withelectrolysis that make it unattractive. For example, electrolysisrequires a large consumption of electrical energy and the oxygen gasproduced can contain small amounts of hydrogen gas which must be removedbefore the oxygen can be used. Additionally the concomitant productionof hydrogen along with oxygen during electrolysis presents serioussafety hazards.

In addition to electrolysis of water, processes are available in the artfor producing pure oxygen gas by separating oxygen from a gaseousmixture such as air.

The most widely used oxygen separation process is cryogenic liquifactionand distillation of air. Such cryogenic processes have severaldrawbacks, however; they are energy intensive with overall efficienciesof less than about 35-40 percent and they must be run in plants whosecapacities exceed about 100 tons per day to take advantage of economy ofscale. Because cryogenic units must be quite large to be economicallyfeasible, smaller and/or portable units based on this technology are notavailable. Therefore, when a cryogenic process is used, the resultingoxygen usually must be shipped from a large central production facilityto the end user. In this case the product oxygen is often transported asa liquid in expensive vehicles equipped with cryogenic dewars. The costof the cryogenic process is further increased since the transport andstorage of liquid oxygen is hazardous and thus, special precautions mustbe taken.

In addition to the above described cryogenic processes, oxygen can beseparated from air by means of known electrochemical processes which arebased either on a two-electron reduction of oxygen or a four-electronreduction. For example, U.S. Pat. No. 3,888,749 to Chong, U.S. Pat. No.4,061,554 to Chillier-Duchatel et al and U.S. Pat. No. 4,300,987 toTseung et al, disclose electrochemical processes for separating oxygenfrom air by means of a two-electron transfer. U.S. Pat. No. 3,410,783 toTomter discloses electrochemical processes for separating oxygen fromair by means of either two- or four-electron transfers.

Since the electrical current which must be passed through an electrolytein an electrolytic cell for separating a given amount of oxygen from agaseous mixture is directly proportional to the number of electrons (n)that reduces each molecule of oxygen, a four-electron process requirestwice the amount of current that is required by a two-electron process.For perfectly reversible (ideal) electrochemical cells the voltage isfixed by the thermodynamic relationship:

    ΔG=-nFE                                              (1)

where ΔG is the free energy change, n is the number of electronstransferred per mole of material passing through the cell, F is theFarady (1F=96,490 Coulombs) and E is the reversible, equilibrium cellvoltage. For the separation of oxygen from air ΔG is fixed and isindependent of the method of separation. Since ΔG is fixed in the idealcase, the energy efficiency is 100% regardless of the value of n becausethe voltage varies to compensate for n. Consider, for example, two idealcells, A and B, with n equal to 4 and 2 respectively, where both cellsoperate with a ΔG of 9.6 kilojoules/mole (kj/mole). The total amount ofpower required to electrolyze 1 millimole/second (mmol/s) of material incells A and B is listed below in Table I.

                  TABLE I                                                         ______________________________________                                                                         Power-Watts                                  Cell    n     Voltage     Amps(I)                                                                              (P = E × I)                            ______________________________________                                        A       4     0.025       384    9.6                                          B       2     0.05        192    9.6                                          ______________________________________                                    

Although, as is shown above for ideal cells, while cell A must pass 384amps and cell B must pass 192 amps to electrolyze 1 mmol of material persecond, the total power required is the same for both cells.

Any real situation is somewhat different however because of unavoidablecell resistances and irreversibilities which prevent 100% efficiency.Thus, as is described below in greater detail, when oxygen is separatedfrom air in an actual (non-ideal) electrochemical cell, the cell with alower n value will be more efficient.

For example, in the non-ideal cell the total power is defined byequation (2) below:

    P=E(faradaic)×I+I.sup.2 R(ohmic)                     (2)

Since the four-electron process (n=4) must pass 2 times as much currentas the two-electron process (n=2) to produce a given amount of productin a given amount of time, the ohmic term for the four-electron processwill be four times as large as for the two-electron process.

Thus, when equation (2) is applied to the example set forth above and anactual cell resistance of 0.001 ohms is assumed, the power requirementto produce 1 mmol/sec of material in cell A is 157 j/mmole while thepower requirement is only 46.5 j/mmole in cell B.

In the foregoing discussion it was assumed that equal amounts of productwere produced during equal time periods in cells with the sameresistance. In practice, it is possible to lower the cell resistance byincreasing the area of the electrodes. For example, by allowing cell Ato have four times the electrode area as cell B, and assuming theresistance of cell A is consequently lowered four-fold, the energyrequirements for the two cells (cells A and B) will be the same. Thus,under conditions of equal energy requirements the cell with a lower nvalue has an advantage in cell size. Since the cost of electrochemicalcells scales roughly with electrode surface area the advantage ofproviding a cell using a relatively lower n can be substantial.

A person skilled in the art can design a cell in such a way thatelectrode area, current density, voltage, product output and rate areoptimized to suit a particular need. In general the cell with smaller nvalue will have an advantage in one or more of these parameters.

SUMMARY OF THE INVENTION

This invention relates to a process in which an electrochemical cellprovided in accordance with this invention is used for separating oxygenfrom air or other gaseous mixture.

In one embodiment, the process of this invention comprises the steps ofcontacting the cathode of the electrochemical cell with a gaseousmixture comprising oxygen to thereby reduce oxygen by a single electronto its superoxide ion. Such superoxide ions are transported across thecell from the cathode to the anode through the electrolyte containedtherein. The superoxide ions are oxidized to oxygen at the anode wherethe oxygen is discharged as oxygen gas.

In a second embodiment, the process of this invention comprises thesteps of adding a transition metal complex to the electrolyte containedin the electrochemical cell. Such a transition metal complex that isprovided in accordance with this invention can be reduced at a potentialmore positive than oxygen reduction. A potential is then applied acrossthe cell to reduce the transition metal complex by a single electron atthe cathode to form a complex capable of reversibly binding oxygen. Agaseous mixture comprising oxygen is introduced into the electrochemicalcell to contact the reduced transition metal complex so that oxygen isbound to the complex. The oxygen-containing complex is transported tothe anode where the complex is reoxidized by a single electron at whichtime said complex releases the bound oxygen for recovery.

In both of the above described embodiments of the process of thisinvention, oxygen is separated from air by means of a single electrontransfer.

The electrochemical cells provided in accordance with this inventioncomprise an anode, a cathode and an electrolyte. In one embodiment theelectrolyte is an aqueous electrolyte and the cathode has a coatingthereon and is provided to reduce oxygen to its superoxide ion. Thecoating is relatively impermeable to water while being relativelypermeable to the superoxide ion. The aqueous electrolyte has a pHgreater than about 7 and provides the means for transporting suchsuperoxide ions across the cell from the cathode to the anode where thesuperoxide ions are reoxidized to oxygen and collected.

In another embodiment of the electrochemical cell provided in accordancewith this invention, the electrolyte comprises an aprotic solventcontaining a dissolved salt.

In yet another embodiment of the electrochemical cell provided inaccordance with this invention, the electrolyte is a solid polymerelectrolyte.

DRAWING

These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims and the accompanyingdrawing which is a schematic of one exemplary embodiment of anelectrochemical cell provided in accordance with practice of principlesof this invention for separating oxygen from air by means of a oneelectron transfer.

DETAILED DESCRIPTION

Although the process and apparatus provided in accordance with practiceof principles of this invention are both described below with referenceto the schematic electrochemical cell 10 shown in the drawing, it shouldbe understood that the components of the electrochemical cell comprisingthis invention can be provided in various configurations as are wellknown in the art of cell design. Furthermore, although this invention isdiscussed in terms of only a single electrochemical cell, the apparatusprovided in accordance with this invention can include a plurality ofsuch cells.

The electrochemical cell 10 of this invention includes a cathode 12, ananode 14 and an electrolyte 16 extending between the cathode and anode.Briefly, in accordance with the process of this invention, the cell 10is operated to separate oxygen from air (or from another gaseous mixturecomprising oxygen) by impressing an appropriate potential across theanode and cathode and by introducing air into a chamber 18 in fluidcommunication with the cathode. The air contacts the cathode surfacewhere oxygen in the air is reduced by one electron to its superoxide ion(O₂ ⁻). (Excess air is vented from the chamber 18 via a vent 20 or thelike.) The superoxide ions produced at the cathode migrate into theelectrolyte and travel through the electrolyte under the influence ofdiffusion, convection and electromigration to the anode where such ionsare reoxidized (by one electron) to oxygen. Oxygen is liberated from theanode as oxygen gas and is collected in an oxygen chamber 22 from whichit is withdrawn for use through a vent, such as the vent 24.

The overall process of this invention is shown as the sum of two halfcells by equations 3 and 4 below:

    O.sub.2 +le.sup.- →O.sub.2.sup.- (cathode); and     (3)

    O.sub.2.sup.- →O.sub.2 ↑+le.sup.- (anode)     (4)

As is described below in greater detail, a key feature of the processand apparatus of this invention is the transport of the superoxide ionsfrom the cathode, where they are formed, through the electrolyte to theanode for reoxidation to oxygen, without a significant number of suchsuperoxide ions being reduced further to peroxide. Thus, the apparatusand process of this invention provide for oxygen to be separated fromair by means of a single electron transfer instead of the two- orfour-electron transfers known previously in the art.

This invention is unique in that it can result in higher efficienciesthan were previously achievable with electro separation cells i.e.,electrochemical cells, based on two- or four-electron transfers. It isalso unique in that no expensive electro catalysts are required foreither the anode or cathode. Further, since the superoxide ion is not astrong oxidant, it does not oxidatively attack the electrodes or othercell components as does the peroxide ion used in prior art processes.Additionally, the potentials required at the electrodes of the instantinvention are more reducing than the normal hydrogen electrode (NHE),whereas in the two- and four-electron processes the electrodes are setat more positive potentials. Because the required potentials are lower,the electrodes in the one-electron process of this invention are notexposed to as harsh an environment as in the two- and four-electronprocesses and thus, are less subject to oxidative degradation.

Electrodes contemplated for use in accordance with practice of thisinvention can be carbon in the form of graphite, vitreous or glassycarbon, carbonblack, carbonized cloth, carbon fibers or other forms ofcarbon known in the art. Alternatively, the electrodes can be non-noblemetals, e.g. mercury or lead, conducting inert borides, carbides,nitrides, silicides, phosphides, and sulfides or noble metals, e.g.platinum or gold.

If desired, the cathode can be in the form of a gas diffusion electrodeto increase the electrode surface available for contacting the air andthus, the oxygen contained in the air. The particulars of constructionof such gas diffusion electrodes are well known to those skilled in theart and do not provide any part of the instant invention.

As is described in greater detail below, the electrolytes contemplatedfor use in accordance with practice of this invention can be aqueouselectrolytes, non-aqueous electrolytes or mixtures thereof.

As was mentioned previously, key features of this invention are thereduction of oxygen in air to the superoxide ion at the cathode andinhibiting the superoxide ion from being reduced further to peroxide asit travels from the cathode to the anode through the electrolyte.

In one embodiment of the cell 10 of this invention, an aqueouselectrolyte is provided and the cathode includes a coating 26 (shownschematically in the drawing) that is relatively impermeable to waterwhile being relatively permeable to the superoxide ion.

If an aqueous electrolyte is used and no such coating is provided,peroxide is formed at the cathode, i.e., it is thought that thesuperoxide ion originally formed at the cathode is reduced further onthe cathode surface to peroxide. A discussion of this reaction and ofcoating a mercury electrode with quinoline to prevent peroxide formationcan be found in J. Chevalet et al, ELECTROGENERATION AND SOME PROPERTIESOF THE SUPEROXIDE ION IN AQUEOUS SOLUTIONS, J. Electroanal. Chem.,39(1972), which is incorporated herein by this reference.

In accordance with practice of this invention such cathode coatings canbe provided by adding a compound (hereinafter referred to as asurfactant) to the electrolyte which is capable of being adsorbed fromthe electrolyte onto the cathode surface. When such a surfactant is usedit is desired, although not necessary, that the aqueous electrolyte issaturated with the surfactant. Alternatively, if desired, the coatingcan be a polymer applied directly to the cathode surface. Non-limitingexamples of surfactants comtemplated for use in accordance with thisinvention include quinoline, triphenylphosphine oxide, pyridine andsubstituted pyridines, substituted quinolines, trialkyl amines, thiolsand thioethers, cetyltrialkylammonium salts, benzyltrialkylammoniumsalts and other cationic surfactants, sodium lauryl sulfate, alkylsulfates and sulfonates, alkyl phosphates and phosphonates and otheranionic surfactants, polyethelyene glycols, polypropylene glycols, andother non-ionic surfactants.

Non-limiting examples of polymers that are contemplated for use incoating the cathode are polyvinylpyridine, polyacrylonitrile,polyacrylamide, and their copolymers.

Once the superoxide ion is formed on the cathode and migrates throughthe cathode coating (which is permeable to the superoxide ion) into theaqueous electrolyte it is inhibited from being further reduced toperoxide in accordance with practice of this invention by providing thatthe electrolyte has a pH greater than 10 and preferably greater than 12.For example, having a relatively high pH reduces the probability thatthe superoxide ions will react with protons and undergodisproportionation according to the following reaction:

    2O.sub.2.sup.- +2H.sup.+ ⃡O.sub.2 +H.sub.2 O.sub.2 ; or (5)

    2O.sub.2.sup.- +H.sub.2 O⃡O.sub.2 +HO.sub.2.sup.- +OH.sup.-(6)

Thus, by providing an aqueous electrolyte solution with a pH greaterthan 10 and preferably greater than 12, the superoxide ions which areproduced at the coated cathode travel through the electrolyte to theanode where they are reoxidized by a single electron transfer to oxygen.The oxygen gas liberated at the anode is then collected for use.

In addition to the above described technique of maintaining the aqueouselectrolyte at a relatively high pH to stabilize the superoxide ion,further stabilization can be provided in accordance with this inventionby adding one or more nitriles to the electrolyte. Non-limiting examplesof nitriles contemplated for use in accordance with practice of thisinvention include benzonitrile, propionitrile, butyronitrile,malononitrile, succinonitrile, adiponitrile, cyanoacetate, 2-cyanoethylether, the cyanopyridines, polyacrylonitrile and acrylonitrileco-polymers, polycyanoacrylate and cyanoacrylate co-polymers.

Preferably such nitriles are added to the electrolyte in an amount toprovide at least about 1% by weight of the nitrile to the total weightof the electrolyte solution.

In addition to adding a nitrile to the aqueous electrolyte to stabilizethe superoxide ion, or as an alternative to such nitrile addition, it isthought that Lewis acids can be added to such an aqueous electrolyte forstabilizing the superoxide ion. For example, it is thought that thesuperoxide ion will associate with cations, especially multivalentcations such as Ca++, Ba++, Zn++ and Al+++, making the superoxide ionless susceptible to disproportionation. Preferably such Lewis acids areadded to the electrolyte in an amount sufficient to provide at leastabout a 0.01 molar (M) solution.

In addition to the foregoing techniques for stabilizing the superoxideion in aqueous electrolytes or, as an alternative thereto, thesuperoxide ion can be stabilized by adding organic cations such astetraalkylammonium, alkylpyridinium, phosphonium, cetyltrialkylammonium,alkyltriethanolamine, and quaternized polyvinylpyridines orpolyethyleneimines to the electrolyte. Preferably, such organic cationsare added to the electrolyte in an amount sufficient to provide at leastabout a 0.1 molar solution.

Alternatively or in addition to the foregoing, the superoxide ion can bestabilized in an aqueous electrolyte in accordance with this inventionby adding to the electrolyte certain macromolecules such as, forexample, the crowns and cryptands. Preferably, such crowns and cryptandsare anion binding crowns and cryptands and are added to the electrolytein an amount sufficient to provide at least about a 0.01 molar solution.

Some transition metals, e.g. iron and copper, are known to catalyze thedisproportionation of the superoxide ion in aqueous electrolytes. Sincehowever, it is also known that certain ligands will act to suppresssuperoxide ion disproportionation by such transition metal ions, it iscontemplated, in accordance with this invention, that when an aqueouselectrolyte is used, various ligands can be added to the electrolyte toincrease the stability of the superoxide ion. Ligands contemplated forsuch use include, for example, ethylenediaminetetraacetate,nitrilotriacetate, triphosphate and ethylenediamine and the like.

The amount of such ligands desired to be added to the electrolyte can bedetermined by one skilled in the art based on the amount ofcontaminants, such as iron and/or copper, present in the electrolyte.

Non-limiting examples of aqueous electrolytes contemplated for use inaccordance with this invention include alkali hydroxides (LiOH, NaOH,KOH, RbOH, CsOH), alkaline earth hydroxides (Mg(OH)₂, Ca(OH)₂, Ba(OH)₂),alkali silicates, alkali borates, alkali and alkaline earth phosphates,alkali sulfates, alkali and alkaline earth halides or combinations andmixtures of the above.

When an aqueous electrolyte is included in the electrochemical cellprovided in accordance with this invention it is preferred that theprocess of this invention includes the step of removing carbon dioxidefrom the inlet air. Such carbon dioxide removal prevents precipitationof carbonates and is particularly important when the nature of theelectrolyte would result in precipitation of insoluble carbonates.

As was mentioned above, in another embodiment of this invention theelectrolytic cell 10 includes non-aqueous electrolyte. Such non-aqueouselectrolytes contemplated for use in accordance with practice of thisinvention are high-boiling (b.p. greater than about 100° F.), aproticpolar solvents that contain an inert salt which dissolves to form atleast a 0.1 molar solution. Preferably such solvents are selected fromthose that have little or no toxicity, especially when the productoxygen gas is intended for medical use.

Aprotic high-boiling solvents contemplated for use in accordance withthis invention include, but are not limited to: pyridine,N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile,benzonitrile, quinoline, substituted pyridines, non-limiting examples ofwhich are methylpyridine, t-butylpyridine, di-t-butylpyridine,tri-t-butylpyridine, N-methylpyridinium salts, N-ethylpyridinium salts,and pyridinecarboxamides, also N-methylpyrrolidinone, dipyridylether,butyronitrile, propionitrile, adiponitrile, chlorocarbons, fluorocarbonsand chlorofluorocarbons, perfluorinated amines and perfluorinatedethers.

Salts that are contemplated for use in combination with the aproticsolvents to provide the electrolyte of this invention include, but arenot limited to: tetraalkylammonium halides, where the alkyl groups arepreferably hydrocarbons having 1 to 16 carbon atoms, preferablytetramethyl ammonium chloride, methylpyridinium halides, ethylpyridiniumhalides, tetraalkylammonium sulfates, perchlorates, acetates andtrifluoroacetates, where the alkyl group is a straight chain hydrocarbonpreferably having a length of 1 to 4 carbon atoms, alkali metal acetatesand trifluoroacetates.

When an aprotic solvent electrolyte system is included in theelectrochemical cell of this invention, no coating is required on thecathode because no protons are available to catalyze further reductionof the superoxide ion formed on the cathode. However, to maintain theaprotic solvent free of protons, i.e., to maintain the solvent free ofwater, the inlet air is preferably dried in a drying step before it isintroduced into the cell. Alternatively, or in addition to drying theair before it is introduced into the cell, a drying agent such asmolecular sieves or activated silica can be added directly to theaprotic solvent electrolyte.

Another feature of the present invention is based on the reactions ofcertain transition metal complexes, especially of cobalt, which can bindsuperoxide ions reversibly in accordance with the following one-electrontransfer reactions:

    O.sub.2.sup.- +L.sub.x Co(III).sup.n+ ⃡L.sub.x Co(III)O.sub.2.sup.(n-1)+                                 (7)

and;

    O.sub.2.sup.- +L.sub.x Co(II).sup.n+ ⃡L.sub.x Co(II)O.sub.2 (n-1)+                                                    (8)

where L designates a ligand, x designates the number of such ligandsassociated with a cobalt ion, n is the total charge of the complex, andII and III represent the cobalt ion valance.

In accordance with practice of this invention, if desired, suchtransition metal complexes can be added either to the above-describedaqueous or non-aqueous electrolytes comprising the electrochemical cellof this invention to further stabilize the superoxide ions. Because thebinding of such a superoxide ion by the transition metal complex isreversible, the complex can act to increase the "effectiveconcentration" of the superoxide ion in solution. Said another way, thetotal concentration of bound and unbound superoxide ion in theelectrolyte can be greater than the concentration of superoxide ion thatcan be obtained with only unbound superoxide when such a complex is notused. Having a higher superoxide ion concentration can result in highercurrent densities and smaller electrode surface areas thereby increasingthe efficiency of the process.

When one or more transition metal complexes are used, oxygen is reducedto its superoxide ion at the cathode and both free and bound superoxideions, the relative amounts of which are determined by the bindingconstant of the complex, are transported across the cell to the anodewhere the free superoxide is oxidized to oxygen. Additionally thesuperoxide bound to the complex is released at the anode and is oxidizedto oxygen. The complex, free of the superoxide ion, then returns to thecathode to pick up superoxide ions being newly formed.

Preferably, such transition metal complexes are added to the electrolytein an amount sufficient to provide at least a 0.01 molar solution. Morepreferably at least a 0.1 molar solution is provided.

In another exemplary embodiment of practice of principles of thisinvention, a redox active transition metal complex can be added to theaqueous or non-aqueous electrolyte 16 of the cell 10 of this invention.Although such use of redox active transition metal complexes results ina different mechanism for separating oxygen from air than was describedpreviously, both mechanisms accomplish oxygen separation by means of aone-electron transfer. For example, when cobalt transition metalcomplexes are used, if the characteristics of the ligand are such thatCo(III) is reduced to Co(II) at potentials more positive than oxygenreduction, the reaction at the cathode will be the production of Co(II).The Co(II) complex will then bind the oxygen from the air and the boundoxygen will be transported across the cell to the anode on the Co(II)complex. Because the binding is reversible the bound and unbound oxygencan equilibrate. At the anode the free Co(II) complex is oxidized toCo(III) in which state it can no longer bind oxygen and thus, the oxygenis released at the anode. The cobalt(III) complex then returns to thecathode to complete the cycle.

The above described process of this invention is shown by the equations8, 9 and 10 below: ##STR1##

When redox active transition metal complexes are used, as describedabove, preferably such transition metal complexes are added to theelectrolyte in an amount sufficient to provide at least a 0.01 molarsolution. More preferably, at least a 0.1 molar solution is provided. Itis thought that if less than a 0.01 molar solution is provided theprocess will not be as economical as desired.

In yet another embodiment, the electrochemical cell of this inventioncan include mixtures of aqueous and non-aqueous electrolytes. Forexample, the superoxide ion is thought to have an appreciable lifetimein mixtures of acetonitrile and water. Such mixed solvents can providestability to the superoxide ion that is comparable to the stabilityprovided by non-aqueous solvents while eliminating the necessity to drythe inlet air. Preferably the mixture comprises at least about 1%acetonitrile by weight compared to the total weight of the electrolyte.

When mixtures of aqueous and non-aqueous electrolytes are used, suchmixtures can be treated as described above for stabilizing thesuperoxide ion, for example by adding nitriles, Lewis acids, organiccations, certain macromolecules such as crowns and cryptands and/orligands such as ethylenediaminetetraacetate, nitrilotriacetate,triphosphate and ethylenediamine. Further, if desired, the transitionmetal complexes described above can be added to the mixed electrolyte.

In addition to the above described non-aqueous electrolytes theelectrolyte provided in yet another exemplary embodiment of practice ofprinciples of this invention can be a solid polymer electrolyte. Solidpolymer electrolytes useful in practice of this invention must beresistant to nucleophilic attack and oxidation, must be stable in thepresence of the superoxide ion and have a low resistance to superoxidemigration. Such solid polymer electrolytes contemplated for use inaccordance with this invention include but are not limited to:polyvinylpyridine, polyvinylpyridine-vinylpyridinium salts,polyethyleneimine and alkylated polyethyleneimine, copolymers whosecomponents are chosen from vinylpyridine, vinylpyridinium salts,ethyleneimine, ethylene oxide, propylene oxide, acrylonitrile,cyanoacrylates, methylmethacrylate, methyl acrylate, styrene,divinylbenzene, divinylpyridine, cumene, pyridylisopropylene and maleicanhydride.

In operation of the electrolytic cell 10 of this invention,temperatures, which are generally in the range of from about 0° to 100°C., are limited by the choice of the electrolyte.

Inlet and outlet pressures of the cell 10 may vary from a partial vacuumof about 20 torr to several atmospheres. Preferably the inlet pressurewill be maintained at one atmosphere or ambient pressure and the outletpressure will be maintained from about 5 to about 10 psi above the inletpressure. It is understood that a higher pressure differential acrossthe cell will normally require an increase in the applied potential.Appropriate pressure regulating systems for use on the electrochemicalcell 10 of this invention are known in the art.

In accordance with practice of the invention the rate at which oxygen isseparated from air or other gasous mixture can be controlled byadjusting the flow of air into the cell, the inlet and outlet cellpressure, and either the voltage across the cell or the current density.If desired the oxygen given off at the anode can be cleaned ofcontaminants by methods known in the art.

The following non-limiting Examples illustrate the separation of oxygenfrom air in accordance with the process of this invention.

EXAMPLE 1

A plexiglas electrolytic cell is divided into two compartments by meansof a polyethylene frit and each such compartment is fitted with mercuryelectrodes. Both compartments are filled with 1 Normal (N) NaOH solutioncontaining 1% (by weight) quinoline. A voltage of 0.5 volts is appliedacross the cell and a stream of air which has been depleted of CO₂ bybubbling it through a gas washing bottle filled with 5 molar NaOH isdirected into the cathode, or negative compartment. The oxygen in theair is reduced by one electron to its superoxide ion at the cathode. Thesuperoxide ions formed at the cathode migrate through the quinoline,which has adsorbed onto the cathode to form a coating, and thencethrough the NaOH solution to the anode. The superoxide ions are oxidizedto oxygen at the anode and released from the anode as oxygen gas. Theanode or positive compartment is protected from air, and gas evolvedfrom the anode is collected by displacement of electrolyte in aninverted chamber or by some other means. The gas evolved at the anode isfound to be substantially enriched in oxygen compared with the inletair.

EXAMPLE 2

A glass two compartment electrolytic cell is fitted with graphiteelectrodes and filled with dry pyridine containing 0.1 molartetraethylammonium chloride. Air dried by passage over anhydrous calciumsulfate and activated molecular sieves is bubbled into a first electrodecompartment. A potential of 0.5 volts is applied to the electrodes withthe electrode in the first compartment held negative with respect to theelectrode in the second compartment. The oxygen in the air is reduced byone electron on the electrode in the first compartment (the cathode) toits superoxide ion. The superoxide ions formed at the cathode travelthrough the electrolyte to the electrode, i.e., the anode in the secondcompartment. The superoxide ions are oxidized to oxygen at the anode andreleased from the anode as oxygen gas. The second or anode compartmentis fitted with a suitable means for collecting any gas evolved at theanode in such a manner that it is not mixed or contaminated with air.The collected gas evolved at the anode is found to be substantiallyenriched in oxygen compared to the air introduced into the first cathodechamber. The evolved oxygen gas is cleaned of entrained pyridine bypassage over activated charcoal.

The above descriptions of preferred embodiments of an apparatus andprocess for separating oxygen from air are for illustrative purposes.Because of variations which will be apparent to those skilled in theart, the present invention is not intended to be limited to theparticular embodiments described above. The scope of the invention isdefined in the following claims.

What is claimed is:
 1. A process in which an electrochemical cell isused to separate oxygen from a mixture of gases, the process comprisingthe steps of:contacting the cathode of an electrochemical cell with agaseous mixture comprising oxygen to thereby reduce such oxygen by asingle electron to its superoxide ion; transporting such a superoxideion across the cell from the cathode to the anode through an electrolytecontained therein; and oxidizing the superoxide ion to oxygen at theanode where said oxygen is discharged as oxygen gas.
 2. The processaccording to claim 1 wherein the electrolyte is an aqueous solutionhaving a pH greater than 7 and the cathode has a coating thereon, thecoating being relatively impermeable to water and relatively permeableto the superoxide ion.
 3. The process according to claim 2 wherein thecoating comprises quinoline.
 4. The process according to claim 2 whereinthe electrolyte contains acetonitrile.
 5. The process according to claim2 wherein the coating is a polymer.
 6. The process according to claim 5wherein the polymer is selected from the group consisting ofpolyvinylpyridine, polyacrylonitrile, polyacrylamide and co-polymersthereof.
 7. The process according to claim 2 wherein the aqueoussolution has a pH greater than about
 10. 8. The process according toclaim 1 wherein the electrolyte is an aprotic, polar solvent containinga salt which is capable of dissolving in the solvent to provide at leasta 0.1 molar solution.
 9. The process according to claim 8 wherein thesalt is selected from the group consisting of tetraalkylammonium saltsand alkylpyridinium salts.
 10. The process according to claim 1 whereinthe electrolyte is a solid polymer.
 11. The process according to claim 1wherein the electrolyte contains a transition metal complex which iscapable of stabilizing the superoxide ion against disproportionation.12. The process according to claim 11 wherein the transition metalcomplex is a cobalt complex which is primarily in the II oxidation stateat the oxygen/superoxide reversible potential.
 13. The process accordingto claim 1 wherein the electrolyte contains a Lewis acid for stabilizingthe superoxide ion against disproportionation.
 14. The process accordingto claim 1 wherein the electrolyte contains a nitrile for stabilizingthe superoxide ion against disproportionation.
 15. The process accordingto claim 1 wherein the cathode is a gas diffusion electrode.
 16. Anelectrochemical cell for use in separating oxygen from a mixture ofgases comprising oxygen, the cell comprising:a cathode having a coatingthereon and capable of reducing oxygen in the gaseous mixture to itssuperoxide ion, the coating being relatively impermeable to water andrelatively permeable to superoxide ion; an anode at which such asuperoxide ion is reoxidized to oxygen; and an aqueous electrolytehaving a pH greater than about 7 the electrolyte providing the means fortransporting such a superoxide ion across the cell from the cathode tothe anode.
 17. An electrochemical cell as claimed in claim 16 whereinthe coating comprises quinoline.
 18. An electrochemical cell as claimedin claim 16 wherein the cathode comprises mercury.
 19. Anelectrochemical cell as claimed in claim 16 wherein the cathodecomprises mercury and the coating comprises quinoline.
 20. Anelectrochemical cell as claimed in claim 16 wherein the aqueouselectrolyte has a pH greater than about
 12. 21. An electrochemical cellas claimed in claim 16 wherein the coating comprises a polymer.
 22. Anelectrochemical cell as is claimed in claim 21 wherein the polymer isselected from the group consisting of polyvinylpyridine,polyacrylonitrile, polyacrylamide and copolymers thereof.
 23. Anelectrochemical cell as is claimed in claim 16 wherein the aqueouselectrolyte contains a transition metal complex which is capable ofstabilizing the superoxide ion against disproportionation.
 24. Anelectrochemical cell as claimed in claim 16 wherein the electrolytecontains a Lewis acid for stabilizing the superoxide ion againstdisproportionation.
 25. An electrochemical cell as claimed in claim 16wherein the electrolyte contains a nitrile for stabilizing thesuperoxide ion against disproportionation.
 26. An electrochemical cellfor use in separating oxygen from a mixture of gases comprising oxygen,the cell comprising:a cathode capable of reducing oxygen in the gaseousmixture to its superoxide ion; an anode at which such superoxide ionsare reoxidized to oxygen; and an electrolyte comprising an aproticsolvent containing a dissolved salt, the electrolyte providing the meansfor transporting such a superoxide ion across the cell from the cathodeto the anode.
 27. An electrochemical cell as claimed in claim 26 whereinthe salt is selected from the group consisting of tetraalkylammoniumsalts and alkylpyridinium salts.
 28. An electrochemical cell as claimedin claim 26 wherein the dissolved salt forms at least a 0.1 molarsolution with the aprotic solvent.
 29. An electrochemical cell for usein separating oxygen from a mixture of gases comprising oxygen, the cellcomprising:a cathode capable of reducing oxygen in the gaseous mixtureto its superoxide ion: an anode at which such a superoxide ion isreoxidized to oxygen; and a solid polymer electrolyte, the polymerelectrolyte providing the means for transporting such a superoxide ionacross the cell from the cathode to the anode.
 30. A process in which anelectrochemical cell is used to separate oxygen from a mixture of gases,the process comprising the steps of:adding a transition metal complex tothe electrolyte contained in the cell, such a transition metal complexcapable of being reduced at a potential more positive than oxygenreduction; providing a potential across the cell to reduce thetransition metal complex at the cathode to form a complex capable ofreversibly binding oxygen; introducing a gaseous mixture comprisingoxygen into the electrochemical cell to contact the reduced transitionmetal complex so that oxygen is bound to said complex; and transportingthe oxygen-containing complex to the anode where the complex isreoxidized at which time said complex releases the bound oxygen forrecovery.
 31. A process in which an electrochemical cell is used toseparate oxygen from a mixture of gases, the process comprising thesteps of:adding a transition metal complex of the formula L_(x)Co(III).sup.(n-1)+ to the electrolyte of the electrochemical cell, whereL designates a ligand, x designates the number of such ligandsassociated with the Co ion, III represents the Co ion valence and n isthe total charge of the complex, the ligand selected to provide thatCo(III) is reduced to Co(II) at potentials more positive than oxygenreduction; providing a potential across the cell to thereby reduce theCo(III) to Co(II) at the cathode to provide a complex of the formulaL_(x) Co(II)^(n+) ; introducing a gaseous mixture comprising oxygen intothe electrochemical cell to contact the L_(x) Co(II)^(n+) complex sothat such oxygen is bound to the complex; and transporting the oxygencontaining complex to the anode where the Co(II) comprising the complexis oxidized to Co(III), at which time the complex releases the boundoxygen for recovery.